Image-forming processes in which electrographic toner is attracted to latent electrostatic charge patterns are well-known in the art.
A fundamental limitation of conventional toning systems is the limitation of image amplification gain. When electrographic toning and optical density formation occur simultaneously, density delivered per electrostatic charge unit cannot be increased without limits. A practical upper value for density deliverable per charge is the primary reason that the photographic speed of conventional electrophotographic systems have been limited to about two orders of magnitude less than that of silver halide systems.
In the past, there have been efforts to form images by developing an electrostatic latent image with a developer containing a reactant and then, through suitable chemical image transfer to a receiver, reacting this chemical image with another reactive substance to form a colored reaction product, such as a dye. For example, U.S. Pat. No. 3,508,823 describes a method for forming an image by heating toners containing dithiooxamide compounds in the presence of a receiver element containing a cobalt salt. However, there is no apparent increase or amplification of photographic speed in those processes involving reactive toners which are made visible in subsequent color forming reactions.
U.S. Pat. No. 4,171,221 describes photosensitive elements comprising a cobalt(III) complex and a chelating compound which elements may be imagewise photoexposed and processed by heating. This produces a visible image by a sequence of steps exhibiting an "internal gain"-that is to say that the amount of image dye produced is greater than that predicted from the number of photons received during photoexposure. Among the disclosed chelating compounds, there may be mentioned 1-(2-pyridylazo)-2-naphthol (PAN) and 4-(2-pyridinylazo)-1,3-benzenediol (commonly referred to as pyridylazoresorcinol or simply PAR). The image-forming reaction is triggered by a photoactive compound, for example a photoreductant which provides cobalt(II) ions to the system.
U.S. Pat. No. 4,201,588 describes photosensitive elements having a radiation-sensitive layer capable of generating an amine. The layer contains a reducible cobalt(III) complex containing ammine or amine ligands and a photoreductant. The released amine is then reacted with another compound in an image-recording layer to form a visible image. An example of such an image-recording layer is a diazo layer coated on a separate support which is laminated to the exposed photosensitive element and heated to form an azo dye image. Among other amine-sensitive image-forming compounds specified is o-phthalaldehyde. This image is formed by phthalaldehyde-cobalt(III) complex-quinone (PACQ) chemistry which again displays internal gain because ammonia is released during dye formation and is then available to be recycled for further dye formation.
U.S. Pat. No. 4,307,168 describes a process in which an electrographic image is formed using a toner whose particles contain a catalyst comprising a metal of Group (VIII) or (IB) of the Periodic Table. The toner image is then amplified using a high gain chemical redox amplification composition. In Example 12 the toner particles are doped using a cobalt(II) compound. The toner image is amplified by heating in contact with a processing sheet containing cobalt(III) tris(ethylenediamine)trifluoroacetate and pyridylazoresorcinol. (The reference in this Example to "cobalt(II)" at column 18, line 16 is an obvious error especially in view of the reference to Research Disclosure Item 14614 referred to at column 18, lines 18-19. It should read "cobalt(III)".) By way of summary it can be said that the cobalt/pyridylazoresorcinol reaction is triggered directly by cobalt(II) ions.